Look at Atlas, South Bend, and Aamco/Delta, and you won't go much wrong. In no particular order.
All 7 inch machines (about as small as you usually find without going hand operated) and while each has different strong and weak points, I would suggest that they are on pretty even terms!
I have owned a South Bend (gave to a friend I actually like!) and kept an Aamco, not because I thought it was better, but because I had found some of the original accessories for it. And, I had that first!
Anyways, the South Bend, had Acme threaded screws while the Aamco/Delta unit uses a standard thread. You can decide if you think one is decidedly better than the other. Other than that, some, but not all, South Bend shapers, had an oil pump to keep them lubed up.
Compared to the Atlas Lathes, the Atlas Shapers were from an earlier era, where cast iron was cheap and available! The Atlas shaper and Mill, were both designed before the penny pinching really started, compared to the later lathes. They still used to much Zamack in the small bits, but were pretty solid little shop toys!
Really, buy the one that appears to be in the best, original condition (as opposed to a pint of paint rebuild)
If you want smaller, look at Martin Model's casting set for the hand shaper! Can be powered with a little jiggery-pokery, but intended to be operated by hand.
Shapers are mesmerising to watch work, and I have whiled away my share of time, just watching it surface off a block of whatever i had available!